The present invention relates to fiber optic networks, and more particularly to fiber optic dense wavelength division multiplexers.
Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Multiple signals and/or wavelengths may be transmitted simultaneously over the same optical fiber. For purposes of this discussion, a signal is a single stream of logical information carried by an optical fiber and a compound signal is the totality of all such simultaneously transmitted signals. If each signal carried in a particular direction along an optical fiber corresponds to a separate wavelength, then the network is a wavelength multiplexed system.
A crucial feature of a wavelength-multiplexed fiber optic network is the separation of a compound optical signal into its component single-wavelength signals, or xe2x80x9cchannels,xe2x80x9d typically by a dense wavelength division multiplexer. This separation must occur for the exchange of wavelengths between signals on communications xe2x80x9cloopsxe2x80x9d within networks to occur. As the demand for information carrying capacity over existing fiber networks continues to grow, the performance constraints on optical network components continue to become more severe.
One potential difficulty with optical channel separators in wavelength multiplexed systems with dense channel packing is imperfect optical separation or add-drop performance as a result of imperfections in optical components. Such imperfections would include unwanted reflections of light at nominally transparent components, unwanted transmissions of light through nominally reflective components, and light scattering from surface roughness of nominally flat surfaces. All such imperfections lead to stray light that can propagate in random and uncontrollable directions. The presence of this stray light can cause signals to propagate along undesired and incorrect pathways, thereby causing imperfect isolation of one set of signals from another. Such imperfect channel separation can cause signals to branch to incorrect portions of the network and can ultimately lead to contamination, for instance, of a signal comprising a particular wavelength channel with spurious incorrectly routed signals carried on the same channel.
As an example, FIG. 1 shows a schematic representation of optical channels as they are transmitted through a dense optical channel separator. A three-port channel separator 101 initially separates an initial optical input signal 103, comprised of a plurality of optical channels, into two sub-signals, a first sub-signal 104 comprised primarily of xe2x80x9coddxe2x80x9d channels and a second sub-signal 105 comprised primarily of xe2x80x9cevenxe2x80x9d channels, respectively. Histograms attached to each signal and sub-signal represent, hypothetically, the intensities of the channels comprising each. If the channel separator 101 is perfect then the sub-signal 104 is comprised only of odd channels and the sub-signal 105 is comprised only of even channels. However, since, in general, the separator 101 is not perfect, there is some xe2x80x9cleakagexe2x80x9d of even channels into sub-signal 104 and of some odd channels into sub-signal 105. Depending upon the level of isolation between sets of channels required by the wavelength multiplexed system, these spurious channel signals could provide unacceptable cross talk if, for instance, new sets of even and odd channels, respectively, are subsequently added to the sub-signals 104 and 105, as illustrated in FIG. 1.
Accordingly, there exists a need for a mechanism for minimizing channel leakage in a dense wavelength division system. The mechanism should be easy to manufacture and be cost effective. The present invention addresses such a need.
The present invention provides a dense wavelength division multiplexer for separating an optical signal into optical channels which minimizes channel leakage. The multiplexer includes at least one channel separator optically coupled to the inputting mechanism, and at least one dense optical channel comb filter optically coupled to the channel separator. In one embodiment, the dense optical channel comb filter includes: a lens optically coupled to the channel separator disposed at a distance equal to the focal length of the lens, a first birefringent wedge optically coupled to the lens at a side opposite to the channel separator, a second birefringent wedge optically coupled to the first birefringent wedge at a side opposite to the lens, and a non-linear interferometer optically coupled to the second birefringent wedge at a side opposite to the first birefringent wedge. The comb filter of the present invention provides an improved efficiency of separation of channels when used to supplement an optical channel separator. The present invention also provides a higher tolerance to drifts than is provided by conventional optical filters due to the increase in the widths of the pass bands. The materials required to manufacture and implement the dense optical channel comb filter in accordance with the present invention are readily available. The present invention thus does not require special or expensive materials or processes. It is thus cost effective.